This invention relates to a vane type compressor used for an automobile air-conditioner, and more particularly to a means for controlling the back pressure of vanes, which is suitably used to improve the performance and durability of such compressors.
In general, a vane type compressor is provided with a rotor on which a plurality of vanes are mounted so as to be movable outward and inward in vane grooves formed in the rotor. This rotor is disposed in a fixed cam ring, so that the vanes slide on the inner surface of the cam ring. Front and rear plates are disposed on both sides of the rotor. A plurality of independent compression chambers are defined by these plates, the inner surface of the cam ring, the outer surface of the rotor and adjacent vanes. The compression chambers change in volume as the rotor rotates, whereby suction and subsequent compression are conducted.
When this compressor is used for a refrigeration cycle, a coolant fed back to the compressor flows into the compression chambers via a suction port formed in the front plate. The coolant which is compressed there to a discharge pressure is discharged into a pressure chamber including therein an oil separator via delivery or discharge ports and discharge valves provided on the cam ring. Only coolant from which oil is separated by the oil separator is delivered to the refrigeration cycle.
The oil (lubricating oil) which is separated from the coolant in the oil separator and which is under the discharge pressure is temporarily stored in a bottom portion of the chamber and then introduced in a pressure-reduced state into a bottom portion of each vane groove due to a difference between the internal pressures in the pressure chamber and compression chamber via an oil supply passage and a spiral throttle inserted therein. The oil in the bottom portion of each vane groove is supplied as a lubricating oil for sliding parts of the compressor, and also as the force (which will hereinafter be referred to as the vane back pressure) for pressing the vanes against the inner circumferential surface of the cam ring, that is, the cam face. Accordingly, the contact pressure of the vanes against the cam face is obtained owing to the force based on the vane back pressure, the force of a gas working on the ends of the vanes and the inertial force, such as the centrifugal force occurring due to the rotation of the rotor. When the rotational speed of the compressor and the pressure conditions therein are constant, all the vanes are pressed against the cam face at the same back pressure. If the vane pressure is constant, the vane tip-pressing force Ft varies depending upon an angle .theta.R of rotation of the rotor which is measured from the mid-point of one of a pair of arc portions of the cam face which are positioned symmetrically with respect to its center. For example, when a ratio of the vane back pressure Pb to the discharge pressure Pd in the compressor, i.e. Pb/Pd is 0.5, Ft, which is at a substantially constant level when the angle .theta.R of rotation of the rotor is not more than 130.degree., decreases suddenly when .theta.R exceeds 130.degree.. When .theta.R is in the vicinity of 160.degree., at which a vane end comes to the discharge ports of the cam ring, Ft increases suddenly. When .theta.R further increases, Ft decreases. When Pb/Pd is 0.5, Ft is not negative, i.e., the vane is not separated from the cam ring. However, when .theta.R is less than 130.degree., Ft may be as high as 9 kg.f, so that frictional loss at the vane end is relatively high. This causes the shaft input in the compressor to increase. Therefore, to reduce frictional loss, it is preferable to reduce Ft by setting Pb/Pd at a lower level. If Pb/Pd is reduced simply, for example, if Pb/Pd is set to 0.43, Ft.apprxeq.0 when .theta.R=158.degree., and Ft&lt;0 when 173.degree..ltoreq..theta.R.ltoreq.180.degree.. In this case, the vane end is separated from the cam ring, and this so-called chattering phenomenon occurs. When this chattering phenomenon occurs, abnormal sounds occur. Moreover, the vanes and cam ring wear abnormally, and the high-pressure gas in a preceding compression chamber defined by the adjacent vanes flows back to a subsequent compression chamber defined by different adjacent vanes, so that the adiabatic efficiency of the compressor as a whole decreases.
Means for preventing vane separation from a cam face in a vane pump is disclosed in U.S. Pat. No. 3,781,145, in which the vane separation is prevented by causing, by the inward movement of a vane in the vane groove, fluid in a vane groove bottom portion to flow through orifices formed in the vane and by creating thereby a higher pressure at the inner end of the vane than at the outer end. Differential pressure between the pressure at the inner end and that at the outer end of the vane presses the vane tip surely on the cam face thereby preventing chattering. Further, the patent discloses a relief port, formed in a front plate for limiting undesirable vane force and resultant undesirable wear, which communicates with the inner end of the vane groove in the region in which excessive or unnecessary pressure would otherwise exist, and limits the vane separation prevention force in a limited region.
According to the U.S. patent, it is necessary to form precise orifices in every vane as shown in FIG. 2, which makes the vanes complicated in construction and not easy in manufacturing.